Downcomer flow control

ABSTRACT

A shell and tube type vapor generator is disclosed in which the vaporizable liquid supply to the evaporator region of the apparatus is automatically regulated in response to load changes on the unit. The vapor generator incorporates a plurality of conduits arranged to conduct liquid from the downcomer reservoir to the evaporation region. The conduits are arranged in such a way that the number of conduits conducting liquid from the reservoir to the evaporator is variable according to the load demand on the unit.

Q United States Fatent [151 3,653,363 Romanos 1 Apr. 4, 1972 [54] DOWNCOMER FLOW CONTROL 3,503,440 3/1970 Romanos ..l22/34 X [72] Inventor: Nicholas D. Romanos, Chattanooga, Tenn. Primary Examiner Kenneth w. Sprague [73] ,Assignee; Combustion Engineering, In Wi d Att0rney-Carlton F. Bryant, Eldon l-l. Luther, Robert L. 01-

Conn. son, John F. Canney, Richard H. Berneike, Edward L. Kochey, Jr. and Lawrence P. Kessler [22] F|1ed: Dec. 10, 1970 211 App1.No.: 96,680 [571 ABSTRACT A shell and tube type vapor generator is disclosed in which the 52 us. CI ..122/32 122/34 vaporizable liquid SuPPlY the evapmmr egion of the v- [51] Int Cl u paratus is automatically regulated in response to load changes [58] Field of Search ..122/32 33 34 the unit- The Benemor incorpmtes a plurality 0f conduits arranged to conduct liquid from the downcomer 56] Refer'ences Cited reservoir to the evaporation region. The conduits are arranged in such a way that the number of conduits conducting liquid UNl D AT PATENTS from the reservoir to the evaporator is variable according to the load demand on the unit. 3,147,743 9/1964 Romanos ..122/32 3,447,509 6/1969 Sprague 122/32 10 Claims, 3 Drawing Figures DOWNCOMER FLOW CONTROL BACKGROUND OF THE INVENTION Shell and tube vapor generators typically comprise a bundle of heat exchange tubes adapted to conduct a heating medium enclosed within a pressure shell. A cylindrical baffle plate normally surrounds the tube bundle in spaced relation from the wall of the shell to define an interior, axially extending evaporator region and an annular downcomer flow passage. A feedwater inlet nozzle communicates with the downcomer annulus to admit vaporizable liquid to the unit and an opening is provided at the bottom of the cylindrical bafile plate to effect fluid communication between the downcomer annulus and the evaporator region.

In vapor generators of the described type, circulation of the vaporizable liquid through the evaporator region of the shell is induced by the hydrostatic head created by the body of liquid contained in the downcomer annulus. Operating experience with such apparatus indicates that the flow of vaporizable liquid is quite susceptible to instabilities. While the exact cause of such flow instabilities is uncertain a number of conditions have been found to contribute to their existence. These include: (a) flow restrictions or changes in the flow path within the tube bundle; (b) restrictions at the evaporator outlet; (c) changes in temperature in the liquid entering the evaporator region and (d) restrictions in the downcomer flow passage. Of these, the last mentioned condition is believed to have a more pronounced affect on flow instability than the others, especially when the discharge opening defined at the bottom of the downcomer annulus presents a flow area larger than that necessary to match the flow required in the evaporator region as determined by load demand on the unit.

It is believed that when this condition exists, changes in the density of fluid in small, localized pockets within the evaporator are readily displaced with greater density liquid. When the liquid expands, local increases in fluid pressure can thus occur whereby disuniformities in fluid circulation will result. As the number and intensity of such changes increase major flow instabilities are created.

Flow instabilities of the descirbed type have been somewhat alleviated in the past by providing flow restrictions of accurately predetermined magnitude in the downcomer annulus so as to regulate the flow of liquid to the evaporator region. Vapor generators so treated were of much smaller capacity than those now contemplated for use in power plant installations and the solution has little effect on the problem as presented in these larger capacity units. Additionally, this solution makes no provision for changes that occur in the operating conditions of a unit as load demand changes due to the fact that no provision is made for adjusting the restrictions, and thereby the rate of liquid supply, in the response to changes ofload demand.

It is to the solution of this problem, therefore, that the present invention is directed.

SUMMARY OF THE INVENTION According to the present invention means are provided for automatically adjusting the amount of vaporizable liquid supplied to the evaporator region of a shell and tube vapor generator from the downcomer annulus thereof in response to changes in load conditions within the unit. In furtherance of the invention reliance is placed on the fact that, in apparatus of the type involved, changes in operating conditions are manifest by changes in the level of vaporizable liquid contained within the downcomer annulus. Thus, as load conditions within a unit increases the level of the liquid body within the annulus will rise proportionately. Conversely, when load demand decreases, the liquid level within the annulus will drop.

The invention contemplates providing a plurality of upstanding conduits within the downcomer annulus, which are adapted to conduct vaporizable liquid from the annulus to the evaporator region of the shell. The conduits are open at both ends with the lower ends being disposed in fluid communication with the evaporator region. The upper end of the conduits, on the other hand, are located at different elevations in the downcomer annulus, the arrangement being such as to provide at each level assumed by the liquid within the annulus an amount of conduit area necessary to conduct an amount of vaporizable liquid to the evaporator region that corresponds to that required to maintain stable fluid circulation at the respective conditions of load. Thus, as load conditions on the unit increase or decrease, a greater or smaller number of conduits are submerged in the liquid such that an alterred amount of conduit flow area will be caused to effect communication between the reservoir and the evaporator thereby to adjust the supply of vaporizable liquid to the latter.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an elevational section of a vapor generator constructed according to the invention;

FIG. 2 is plan section taken along line 22 of FIG. 1;

FIG. 3 is a development of a portion of the vapor generator of FIG. 1;

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The drawings illustrate a shell and tube type vapor generator 10 comprising a vertically elongated cylindrical pressure shell 12 that is closed at its opposite ends by upper and lower domed closure members, 14 and 16 respectively. Within the interior of the shell a pair of axially spaced, transversely extending tube sheets, 18 and 20 respectively, are integrally attached to the interior shell wall and divide the shell into axially spaced chambers indicated as heating fluid inlet chamber 22, heating fluid outlet chamber 24, and evaporator chamber 26. A plurality of straight, fluid conducting heat exchange tubes 28 extend through the evaporator chamber 26 having their ends connected between the two tube sheets and communicating with the respective heating fluid inlet and outlet chambers 22 and 24. Nozzles 30 and 32 communicate with the respective inlet and outlet chambers for circulating heating fluid from a source (not shown) through the vapor generator 10.

The heat exchange tubes 28 are arranged, as shown, in an axially extending tube bundle whose outer periphery is concentrically spaced from the interior surface of the shell 12. An axially elongated, cylindrical baffle 34 surrounds the tube bundle in spaced relation from the shell wall thereby to form an annular space between the wall and the baffle defining downcomer reservoir 36. A feedwater inlet nozzle 38 penetrates the wall of shell 12 and connects with a ring manifold 40 disposed above the upper end of the baffle 34 for supplying feed liquid to the downcomer reservoir 36. Feed liquid is distributed about the circumference of the reservoir by discharging through circumferentially spaced openings 42 depending from the lower side of the manifold 40.

The downcomer reservoir 36 is caused to communicate with the chamber 26 as hereinafter more fully described supplying the latter with vaporizable liquid that flows in heat exchange relation with the heating fluid flowing through tubes 28. Within the chamber 26 there is defined a lower liquid space 44 and an upper vapor space 48 occupied by the flowing fluid in its various states. The combined hydrostatic and dynamic forces imposed by the flowing fluid establish a specific liquid level 46 in the reservoir 36 for a particular set of operating conditions. As conditions change the position of level will vary between lines 46a and 461;.

In the described embodiment of the invention the vapor space 48 is provided with a number of axially spaced, horizontally extending baffle plates 50 defining a vapor flow path in which the vapor is conducted in cross-flow relation to the tubes in order to enhance heat transfer between the heating medium and the flowing vapor. In the vapor space 48, vapor will extract heat from the heating medium to be dried and to be imparted with a predetermined degree of superheat.

and the shell wall in the vapor space 48 to define an annular flow passage that communicates at its upper end with the vapor space. A vapor outlet nozzle 56 penetrates the wall of shell 12 to communicate with the flow passage 54 thereby to conduct superheated vapor from the vapor generator to a point of use.

According to the present invention, means are provided in the herein described vapor generator 10 for automatically regulating the flow of vaporizable liquid from the downcomer reservoir 36 into the evaporator chamber 26 in response to load changes on the unit. This means comprises a plurality of vertically elongated conduits 58 that are disposed in circumferentially spaced relation about the downcomer reservoir 36. The conduits 58 may be attached to the baffle 34 by welding or otherwise suitably secured within the space. The conduits 58 are open ended tubulous members with the lower ends 60 thereof being disposed in substantially coplanar relation and spaced above the upper surface of the lower tube sheet to place the conduits in fluid communication with the body of liquid 44 in the evaporator chamber 26.

The arrangement of the conduits is such that vaporizable liquid from the downcomer reservoir 36 will be discharged into the evaporator region at a rate of flow that corresponds to that required to establish stable fluid circulation within the unit at all load conditions. In order to produce the effect, the conduits are formed of different lengths with their upper ends, 62a, 62b, 62c, etc. disposed at different elevations within the reservoir. In this way a different number of tubes, and concomitantly a different amount of flow area, is caused to effect communication between the downcomer reservoir and the evaporator region of the chamber 36 as the liquid level within the reservoir changes. Preferably, the conduits 58 are arranged in groups as shown in FIG. 3 with each group consisting of one conduit of each of the lengths provided. Alternatively, but less desirably, conduits of equal length can be grouped together and the respective groups circumferentially spaced from one another about the reservoir 36.

The bottom of the downcomer reservoir 36 is preferably closed by means of a horizontally disposed annular plate 64 that extends between the lower end of the cylindrical baffle 34 and the inner surface of the shell 12. The plate 64 is provided with sealed openings that pass the lower ends 60 of the conduits 58 whereby to prevent liquid flow to the chamber 26 except through the interior of the conduits. Alternatively, it may be found to be desirable in some installations to provide additional openings in the plate 64 or even to omit the plate entirely whereby a portion of the liquid flow from the reservoir 36 to chamber 26 will be caused to bypass the conduits 58. In vapor generators providing for liquid bypass the upper ends 62 of conduits 58 will be disposed at levels somewhat higher than those of the preferred embodiment wherein liquid bypass is not employed.

The operation of the herein discribed invention is as follows. Heating fluid from a source is admitted to the inlet chamber 22, passed through tubes 28 and discharged from the outlet chamber 24. Coincident with the circulation of heating fluid, vaporizable liquid is admitted to the unit through feedwater nozzle 38 and ring manifold 40 from whence it is discharged into downcomer reservoir 36 to form a body of liquid therein. The so-formed body of liquid will have a level assuming a position somewhere between the lines 46a and 46b depending upon the operating conditions of the vapor generator 10. From the reservoir 36 liquid is supplied to the chamber 26 through the conduits 58, as hereinafter more fully described, where it flows in indirect heat transfer relation with the heating fluid flowing through the tubes 28. The vapor that is generated within the liquid body passes therefrom to the vapor space 48 where it is directed by bafile plates 50in crossflow relation to the tubes 28 thereby to be dried and further heated to some degree of superheat. Heated vapor then flows through the annular flow passage 54 before exiting the unit through the outlet nozzle 56 and conducted to a point of use.

As is well known, circulation of vaporizable fluid in vapor generators of the described type is induced by the hydrostatic head created by the body of liquid contained in the downcomer reservoir 36. Thus, a change in load conditions in the vapor generator will produce an alteration in the flow of vaporizable fluid through the chamber 26 and consequently, a change in the pressure drop along the flow path that will be balanced by the head produced by the liquid body in reservoir 36. At low load conditions the liquid level will therefore assume a position such as that represented by line 46a in the drawings, for example. At full load the level will be in the position represented by line 46b and at intermediate loads the level will assume levels between lines 46a and 46b commensurate with the particular load conditions that prevail.

Thus, for any given load condition a specific level will be assumed by the liquid level in the downcomer reservoir 36. The supply of liquid to the chamber 26 will therefore be dependent upon the number of conduits 58 caused to conduct liquid from the reservoir to the chamber as determined by the position of the liquid level with respect to the upper ends 62 of the respective conduits. Therefore, at low load conditions where the liquid body assumes a level about the line 46a in FIG. 3, only those conduits whose upper ends, indicated as 620, are submerged will conduct liquid to the chamber 26. As load increases and the liquid level rises, the open ends 62b,62c, etc. of additional tubes will be submerged thereby increasing the amount of liquid passed to the chamber. At full load conditions all of the conduits will be caused to conduct liquid so that the maximum rate of liquid supply will then be achieved. While a greater or less number of conduits 58 than those illustrated herein maybe employed without departing from the spirit of the invention, it should be recognized that the sensitivity of liquid regulation will vary directly with the number of conduits of different lengths employed in the unit.

The invention described herein provides a simple, inexpensive means for regulating the supply of vaporizable liquid to the evaporator region of shell and tube type vapor generators. By means of the invention fluid circulation instabilities that are characteristic of the operation of vapor generators of this type are alleviated or avoided since the supply of liquid to the evaporator region can be accurately regulated over the full load range of unit operation.

It will be understood that various changes in the details, materials, and arrangement of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. in heat exchange apparatus of the type wherein vapor is generated by the indirect transfer of heat between a fluid heating medium and a vaporizable fluid including a pressure shell, a bundle of heat exchange tubes for conducting the heating medium within said pressure shell, baffle means within the shell dividing its interior into a downcomer reservoir and an evaporator region, means for supplying vaporizable liquid to said downcomer reservoir to form a body of liquid therein having a depth that corresponds to the load conditions on the apparatus, the improvement comprising a plurality of conduits disposed in said downcomer reservoir, said conduits having their discharge ends in fluid communication with said evaporator region and their inlet ends disposed at different elevations in said reservoir whereby the number of conduits employed to conduct vaporizable liquid from said reservoir to said evaporator region will vary according to the depth of said liquid body.

2. Apparatus as described in claim 1 wherein said downcomer reservoir is an annular space surrounding said evaporator region and said conduits are circumferentially spaced about said downcomer reservoir.

3. Apparatus as described in claim 2 including a plurality of conduits having their inlet ends disposed at each elevation wherein said conduits are arranged in groups and said groups are circumferentially spaced about said downcomer reservoir.

4. Apparatus as described in claim 3 wherein each of said groups comprises a plurality of conduits each having its inlet end disposed at a different elevation in said downcomer reser- V".

5. A vapor generator comprising:

a. a vertically elongated pressure shell;

b. a cylindrical baffle concentrically spaced from the interior wall of said shell dividing said shell into an evaporator region and a concentrically related downcomer reservoir;

c. a plurality of heat exchange tubes extending through said evaporator region;

d. means for circulating heating fluid through said tubes;

e. means for supplying vaporizable liquid to said downcomer reservoir to form a body of liquid therein having a depth that varies in accordance with load conditions on said vapor generator;

f. a plurality of conduits immersed in said body of liquid within said downcomer reservoir, said conduits having their discharge ends in fluid communication with said evaporator region and their inlet ends arranged at different elevations in said reservoir whereby the number of tubes efl'ective to conduct liquid from said reservoir to said evaporator region will vary according to the depth of said liquid body.

6. Apparatus as recited in claim 5 wherein said conduits are upstanding tubulous members having their axes parallel to that of said cylindrical baffle.

7. Apparatus as recited in claim 6 including a plurality of conduits having their inlet ends disposed at each elevation, said conduits being arranged in groups and mutually spaced within said downcomer reservoir.

8. Apparatus as recited in claim 7 wherein each of said groups comprises a plurality of conduits each having their inlet end disposed at a difi'erent elevation.

9. Apparatus as recited in claim 8 wherein said cylindrical bafile surrounds said bundle of heat exchange tubes to define an interior evaporator region and an annular downcomer reservoir.

10. Apparatus as recited in claim 9 wherein said groups of conduits are circumferentially spaced about said downcomer reservoir. 

1. In heat exchange apparatus of the type wherein vapor is generated by the indirect transfer of heat between a fluid heating medium and a vaporizable fluid including a pressure shell, a bundle of heat exchange tubes for conducting the heating medium within said pressure shell, baffle means within the shell dividing its interior into a downcomer reservoir and an evaporator region, means for supplying vaporizable liquid to said downcomer reservoir to form a body of liquid therein having a depth that corresponds to the load conditions on the apparatus, the improvement comprising a plurality of conduits disposed in said downcomer reservoir, said conduits having their discharge ends in fluid communication with said evaporator region and their inlet ends disposed at different elevations in said reservoir whereby the number of conduits employed to conduct vaporizable liquid from said reservoir to said evaporator region will vary according to the depth of said liquid body.
 2. Apparatus as described in claim 1 wherein said downcomer reservoir is an annular space surrounding said evaporator region and said conduits are circumferentially spaced about said downcomer reservoir.
 3. Apparatus as described in claim 2 including a plurality of conduits having their inlet ends disposed at each elevation wherein said conduits are arranged in groups and said groups are circumferentially spaced about said downcomer reservoir.
 4. Apparatus as described in claim 3 wherein each of said groups comprises a plurality of conduits each having its inlet end disposed at a different elevation in said downcomer reservoir.
 5. A vapor generator comprising: a. a vertically elongated pressure shell; b. a cylindrical baffle concentrically spaced from the interior wall of said shell dividing said shell into an evaporator region and a concentrically related downcomer reservoir; c. a plurality of heat exchange tubes extending through said evaporator region; d. means for circulating heating fluid through said tubes; e. means for supplying vaporizable liquid to said downcomer reservoir to form a body of liquid therein having a depth that varies in accordance with load conditions on said vapor generator; f. a plurality of conduits immersed in said body of liquid within said downcomer reservoir, said conduits having their discharge ends in fluid communication with said evaporator region and their inlet ends arranged at different elevations in said reservoir whereby the number of tubes effective to conduct liquid from said reservoir to said evaporator region will vary according to the depth of said liquid body.
 6. Apparatus as recited in claim 5 wherein said conduits are upstanding tubulous members having their axes parallel to that of said cylindrical baffle.
 7. Apparatus as recited in claim 6 including a plurality of conduits having their inlet ends disposed at each elevation, said conduits being arranged in groups and mutually spaced within said downcomer reservoir.
 8. Apparatus as recited in claim 7 wherein each of said groups comprises a plurality of conduits each having their inlet end disposed at a different elevation.
 9. Apparatus as recited in claim 8 wherein said cylindrical baffle surrounds said bundle of heat exchange tubes to define an interior evaporator region and an annular downcomer reservoir.
 10. Apparatus as recited in claim 9 wherein said groups of conduits are circumferentially spaced about said downcomer reservoir. 